Here below, we discuss the drawbacks of the prior art through the special case of the power amplifiers used by radio communications devices, for example radio telephones or PDAs (Personal Digital Assistants).
When designing a radio communications device, it is sought especially to reconcile at least some of the following goals:                the security of the device and especially checks on the electromagnetic compatibility (EMC) of the power amplifier. According to a commonly accepted definition, the term “electromagnetic compatibility” is understood to mean the capacity of a piece of hardware or a system to work in its electromagnetic environment satisfactorily and without itself producing electromagnetic disturbances that would be intolerable for anybody situated in this environment;        the efficiency of the heat control, the temperature of the power amplifier which must be as low as possible so as to improve the temperature behavior and increase the life span of the device;        the compactness of the device since the surface area of the printed circuit used by each of the components of the device must be as small as possible;        simplicity and low cost of manufacture.        
FIG. 1 illustrates an electronic circuit 100 of a second-generation (2G) radio communications device. The circuit 100 has a shielding 11 which can take the form of a metal pan entirely covering the front face 1 of the printed circuit 12 on which the different components are implanted (for example the radio communications module, the power amplifier or amplifiers etc). In the prior art, such a screen 11 is essentially designed to prevent the energy radiated by the power amplifier or amplifiers from going out of the device (in the form of harmonics conveyed by electromagnetic radiation) and from disturbing the other functional blocks of the device or other neighboring apparatuses.
The second generation (2G) radio communications devices generally use high-efficiency non-linear power amplifiers.
FIG. 2 is a partial schematic view in cross-section of the electronic circuit 100 of FIG. 1. In this example, the power amplifier 200 is mounted on a rear face 2 of the printed circuit 12. The power amplifier 200 includes amplifier means 21 encapsulated in a package 22 made out of molded plastic. The package 22 has a metal sole 23 comprising a first face designed to receive the amplifier means 21 and a second face designed to be fixedly joined to the rear face 2 of the printed circuit 12. Conventionally, the package 22 of the amplifier 200 (high-efficiency non-linear power amplifier) is used, on its own, to release a large part of the heat released by the amplifier means 21. This heat C1 is therefore radiated on the rear face 2 of the printed circuit 12.
As illustrated in FIG. 3, a heat sink 31 can be used to obtain better thermal dissipation. Indeed, the heat sink 31 is used to collect a part of the heat released by the amplifier means 21 through the metal sole 23 and then convey and radiate this collected heat C2 on the front face 1 side of the printed circuit 12.
In the example illustrated, the heat sink 31 has a metal base 311 designed to be fixedly joined to the metal sole 23 and two metal arms 213 and 313 that extend perpendicularly to the base 311 and are designed to get engaged in the printed circuit 12. Each of the metal arms 312 and 313 goes through the rear face 2 of the printed circuit 12 and emerges on the front face 1 of the printed circuit 12. Thus the metal base 311 is used to collect a part of the heat released by the amplifier means 21 and the metal arms 312 and 313 enable this collected heat C2 to be discharged on the front face 1 side of the printed circuit 12 through the copper-plated part of the front face 1 of the printed circuit 12.
Third-generation (3G) radio communications devices generally use low-efficiency linear power amplifiers.
At present, the classic techniques (described here above with reference to FIGS. 1 to 3) for heat dissipation in 2G power amplifiers are applied to 3G power amplifiers.
The inventors have noted that the classic techniques mentioned here above are not suited to 3G power amplifiers because the increase in temperature with the output power of the 2G power amplifiers is far from that of the 3G power amplifiers. In other words, the classic techniques mentioned here above cannot be used to adequately and speedily dissipate the heat released by the 3G power amplifiers.
Furthermore, although heat sinks improve thermal dissipation by discharging a part of the heat produced by the power amplifier on the front face side of the printed circuit, there is nevertheless a risk of a glasshouse effect. Indeed, the shielding prevents the discharge of the heat conducted by the heat sinks towards the exterior of the printed circuit (i.e. out of the shielding). This heat C2 is therefore confined to the zone defined between the screen and the front face of the printed circuit.
There is therefore a need to optimize the thermal control of the 3G power amplifiers, especially to maximize the temperature behavior and life span of the radio communications device.